Non-Linear Junction Detection (NLJD) is a well-known technique for detecting electronics that utilize semiconductor (solid-state) junctions. The current state of the art for finding hidden electronics such as electronic eavesdropping devices using this technology has a maximum range of about 2 m, and more typically between 6 in. and 12 in. A bare diode may be viewed as a dipole antenna having a nonlinear junction separating the two antenna elements. The response of a nonlinear junction to an applied voltage follows the IV curve for the junction, and may be described by I=I0(eqV/kT−1) where q is the electronic charge, k is Boltzman's constant, T is the temperature of the device in Kelvin, V is the applied voltage, and I is the current flowing through the junction. The first two terms in the expansion of this expression are: I=I0(qV/kT+½(qV/kT)2), the second term being responsible for generating the second-harmonic (doubled) frequency which is determinative of the RF radiation from the sought electronics. It is this frequency which is detected as an indicator of the presence a nonlinear or semiconductor junction associated with electronics. In the presence of a RF field, the voltage, V, is determined by the applied field (the transmitted power). When it exceeds the bias voltage, current I flows through the device and may be re-radiated. Because the current is a nonlinear function of the applied voltage, the re-radiated energy contains harmonics of the fundamental applied RF frequency. In its simplest form, then, a nonlinear junction detector irradiates an area using frequency f, and detects returning electromagnetic radiation at frequency 2f (and possibly 3f, etc.).
Electronic devices typically contain multiple nonlinear junctions linked by wires or traces to other components. Therefore, energy may couple into and out of the device through multiple paths; moreover, the path(s) into a device may be different than the path(s) out of the device. It is to be noted that powering a device may alter its coupling characteristics (that is, biasing a diode of interest will place a signal farther up the IV curve).
A popular commercial device is the ORION (See, e.g., http://www.tscm.com/orion.html.). The ORION is effective, but has a range of only about 12 in. Simply increasing the transmitted power to several Watts with the hope of increasing the range for locating targets on the ground at several tens of meters has been found to be ineffective because of false positives (self-detection), low sensitivity, and severe attenuation of RF propagation along the ground.
Spread spectrum techniques are commonly used in communications, as they provide high sensitivity for low power requirements. Examples include cell phones (Code Division Multiple Access (CDMA)), and GPS (the latter uses a 50 W transmitter 20,000 km away). Pseudo-random encoding at the transmitter and cross-correlation at the receiver is used to detect and locate extremely weak signals, even in a noisy RF environment (this is how dozens of cell phones can work in close proximity without interfering with one another). Multiple techniques exist, including phase shift keying (PSK), frequency shift keying (FSK), amplitude shift keying (ASK), and the like.